Light beams of high intensity such as laser beams are used for producing a reference line or plane which can be used for aligning purposes. One such use is in the aligning of tracks such as is disclosed in U.S. Pat. Nos. 3,706,284, Plasser et al., issued Dec. 19, 1972, and entitled TRACK WORKING APPARATUS WITH LASER BEAM REFERENCE, and 3,750,299, Plasser et al., issued Aug. 7, 1973, and entitled TRACK APPARATUS WITH LASER BEAM REFERENCE.
Prior art projectors used for aligning purposes have generated planes of light for use as references in several different ways. One apparatus of this type produces a fan by means of a fan lens. In other words, optics is used for the purpose of spreading out the light projected by a laser. Such fans of light created by fan lens also require for use in sunlight conditions the modulation of the projected light which is generally accomplished electronically so as to constantly decrease and increase the intensity to provide the modulation.
Another type of apparatus used is to physically reciprocate the projector so that the light beam is reciprocated from one position to another along the reference plane which thereby presents to the detector a modulated signal as the light reciprocates from one position to the next along the reference plane and over the sensor located on the plane. Such apparatus is disclosed in U.S. Pat. No. 3,823,313, Unema, issued July 9, 1974, and entitled LASER FANNING DEVICE.
Another type of apparatus used for producing a plane of light which has the effect of a modulating light source is to project the light beam against an optical element such as a mirror or pentaprism which is rotated and directs the light beam at a 90.degree. angle to the direction of its original projection. This creates a beam of light that is rotating like a beacon and which has the effect of creating a plane of light which when passed over a sensor gives to the sensor the effect of a modulated signal. This type of apparatus is disclosed in U.S. Pat. No. 3,936,197, Aldrink et al., issued Feb. 3, 1976, and entitled SELF-LEVELING LASER ASSEMBLY.
All of the above described prior art apparatus for generating a modulated plane of light have many limitations, particularly when used to project the reference plane at long distances such as between 1000 and 2000 feet. The apparatus using fan lens and light intensity modulation produces a signal at long distances which is very difficult to detect because of the low intensity. Therefore, sophisticated sensors are required at long distances and in many cases sufficiently sensitive sensors have not been developed which will sense the projected light and not be affected by the sunlight. Further, such apparatus requires complicated fan lenses and means for modulating the intensity of the light source.
The apparatus for reciprocally moving the projector has the disadvantage in that external moving parts are required that eventually wear out and must be replaced. Further, in the use of external moving parts, it is very difficult with the use of such parts to obtain the accuracy required for many uses. In addition, there are vibration and inertia problems in the moving of the entire projector. Also, it is desirable that the projector be reciprocated an extremely small distance in order to avoid the excessive spreading out of the light at the location of the sensor which in many instances may be over 1000 feet from the projector. In spreading the light through such a long distance along the plane, the light intensity is reduced significantly so as to present the same problem as described above in relation to the low intensity of the fan lens type apparatus. Actually moving the projector also presents problems and significant errors created by any play in the parts such as the bearings supporting the projector.
With the rotating beam type of apparatus as above described, many of the same problems and disadvantages as described above are encountered. Foremost among these is the light intensity problem and the problems encountered in the mechanism to mount and rotate the mirrors or pentaprisms.
All of the above apparatus is complicated and cumbersome for many intended uses.